Method of producing a piezoelectric/electrostrictive device and piezoelectric/electrostrictive element

ABSTRACT

A piezoelectric/electrostrictive device is provided, including a pair of mutually confronting thin plate portions, a fixing portion for supporting the pair of thin plate portions, movable portions provided at tip end portions of the pair of thin plate portions and having mutually confronting end surfaces, and piezoelectric/electrostrictive elements disposed on respective thin plate portions. At least both side surfaces of the thin plate portions and the piezoelectric/electrostrictive elements are covered with coating films made of a material with a low thermal expansion coefficient.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of U.S. application Ser. No. 10/448,999,filed May 30, 2003, now allowed, the entirety of which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to a filmy piezoelectric/electrostrictiveelement, a piezoelectric/electrostrictive device having movable portionsthat are operated based on a displacing operation of thepiezoelectric/electrostrictive element, and a production method thereof.Specifically, the present invention relates to apiezoelectric/electrostrictive element that is excellent in temperaturecharacteristic to enable a displacement control with high accuracy athigh temperatures, and that is not subjected to deterioration even athigh temperature and high humidity thereby to enable realization of astable displacing operation over a long term, and further relates to apiezoelectric/electrostrictive device provided with thepiezoelectric/electrostrictive element, and a production method thereof.

BACKGROUND OF THE INVENTION

In recent years, in the fields of optical and precision apparatuses,semiconductor production, and so on, there has been a demand fordisplacement control elements that adjust optical path lengths,positions, etc. in the order of submicrons. In response thereto, therehave been developed piezoelectric/electrostrictive elements that utilizedistortion caused by an inverse piezoelectric effect or anelectrostrictive effect generated upon applying an electric field to aferroelectric body or antiferroelectric body. These displacement controlelements utilizing the electric field induced distortion have featuresthat it is easy to execute a small displacement control as compared witha conventional electromagnetic technique using servomotors, pulsemotors, etc., the mechanical/electrical energy conversion efficiency ishigh so that power saving may be achieved, ultra-precise mounting ismade possible, reduction in size and weight of the product can beachieved, and so on. Therefore, it is expected that application fieldswill be expanded steadily.

As such a displacement control element, for example, JP-A-10-136665discloses a piezoelectric actuator wherein, as shown in FIG. 11, aplate-like member 200 made of a piezoelectric/electrostrictive materialis provided with a hole portion 202, and a fixing portion 204, a movableportion 206, and beam portions 208 supporting them are formed integralwith each other, and further, an electrode layer 210 is provided on thebeam portion 208.

In this piezoelectric actuator, when an voltage is applied to theelectrode layer 210, the beam portion 208 is extended/contracted in adirection of connecting the fixing portion 204 and the movable portion206, so that it is possible to cause the movable portion 206 to performarc-shaped displacement or rotational displacement in the plane of theplate-like member 200.

However, in the piezoelectric actuator disclosed in JP-A-10-136665,since the displacement in the extending/contracting direction of thepiezoelectric/electrostrictive material (i.e. in-plane direction of theplate-like member 200) is transmitted to the movable portion 206 as itis, there has been a problem that an operation amount of the movableportion 206 is small. Further, since the piezoelectric actuator isentirely formed of the piezoelectric/electrostrictive material that isfragile and relatively heavy, it is low in mechanical strength, andinferior in handleability, impact resistance, and moisture resistanceand, in addition, the piezoelectric actuator itself is heavy so thatthere has been a problem that it is susceptible to influence of harmfulvibration (e.g. residual vibration or noise vibration upon high-speedoperation) upon operation.

We have newly developed a piezoelectric/electrostrictive device providedwith movable portions that are operated based on a displacing operationof a piezoelectric/electrostrictive element being a displacement controlelement, and proposed in JP-A-2001-320103 along with its productionmethod, thereby to solve the aforementioned problem. Thispiezoelectric/electrostrictive device comprises a pair of mutuallyconfronting thin plate portions and a fixing portion supporting thesethin plate portions, wherein movable portions are provided at tip endportions of the pair of thin plate portions, and one or morepiezoelectric/electrostrictive elements are disposed on at least one ofthe pair of thin plate portions, and is characterized in that themovable portions have mutually confronting end surfaces, and a distancebetween the end surfaces is greater than a length of the movableportion.

SUMMARY OF THE INVENTION

The present invention has been made to further improve theaforementioned our proposal. Specifically, the aforementioned previouslyproposed piezoelectric/electrostrictive device comprising the thin plateportions having the movable portions at the tip end portions thereof,and the fixing portion supporting them wherein thepiezoelectric/electrostrictive elements are disposed on the thin plateportions, can be preferably used as, for example, an actuator for finelypositioning a head element of a magnetic disk, an optical disk, or thelike, and is an excellent small displacement control element.

However, recently, following the increasing capacity and density ofmagnetic disks and optical disks, there has been raised a demand forfurther improving the limit of positioning accuracy, and it has beenunable to fully satisfy such a demand with the previously proposedpiezoelectric/electrostrictive device as it is.

It has been considered that the limit of positioning accuracy of thepreviously proposed piezoelectric/electrostrictive device is induced bythe use environment. Specifically, when used for the aforementionedpurposes, the change in temperature is large and the temperature becomeshigh in the environment of use. Accordingly, it has been consideredthat, due to the temperature characteristics of the material forming thethin plate portions (vibration plates) or of a piezoelectric orelectrostrictive material, or the like, or caused by the fact that astress remains in piezoelectric/electrostrictive layers of thepiezoelectric/electrostrictive elements due to a difference between atemperature upon production process and a temperature upon use, thepiezoelectric/electrostrictive device does not produce expecteddisplacement, which should follow an applied electric field, anddisplaces largely, for example, and therefore, it is difficult toachieve a small displacement control with ultra-high accuracy.

More specifically, in case of, for example, PZT being the typicalpiezoelectric material, the thermal expansion coefficient thereof is1.4×10⁻⁶/° C., while the thermal expansion coefficient of metal (e.g.ferrous alloy such as various stainless steel or spring steel, copperalloy such as brass or beryllium copper, aluminum alloy such asduralumin) excellent in mechanical characteristic, or high-strengthceramics (e.g. alumina, partially stabilized zirconia), which is used asa material forming the thin plate portions, or an electrode material, is7.5×10⁻⁶/° C. or greater. Therefore, following the change in temperatureof the ambient environment or the element itself, a stress is generatedbetween the piezoelectric material and the electrode material, and thethin plate portions, so that displacement thereof is changed to manifestunexpected displacement.

The reason thereof is that, upon forming the piezoelectric element onthe thin plate portion, the process temperature of about 100 to 150° C.is applied when using thermosetting epoxy adhesive agent that provideshigh adhesion strength, while, the process temperature of 1000° C. orhigher is applied when performing firing for integration. Therefore, ifthe temperature upon use for the aforementioned purpose is around roomtemperature, a stress remains due to that difference in temperature.Accordingly, there has arisen necessity for solving them, and thepresent invention has been reached.

Therefore, an object of the present invention is to solve theaforementioned problems and, in other words, to provide apiezoelectric/electrostrictive device that produces displacementfollowing an applied electric field irrespective of changes in the useenvironment temperature or the element itself, or even upon use at hightemperatures, thereby to enable a small displacement control withultra-high accuracy. As a result of continuing intensive study about amethod for suppressing displacement that becomes larger than a controlvalue following the rise of temperature with respect to thepiezoelectric/electrostrictive element forming thepiezoelectric/electrostrictive device, it has been found that theaforementioned object can be accomplished by means shown below.

Specifically, according to the present invention, there are firstprovided the following two piezoelectric/electrostrictive devices.

The first piezoelectric/electrostrictive device comprises a pair ofmutually confronting thin plate portions, and a fixing portionsupporting the pair of thin plate portions, wherein movable portions areprovided at tip end portions of the pair of thin plate portions, themovable portions have mutually confronting end surfaces, and one or morepiezoelectric/electrostrictive elements are disposed on at least one ofthe pair of thin plate portions, and is characterized in that at leastboth side surfaces of the thin plate portions and the one or morepiezoelectric/electrostrictive elements are covered with coating filmsmade of a material with a low thermal expansion coefficient.

Here, there is no limitation to the low thermal expansion coefficientmaterial as long as it is a material having a smaller thermal expansioncoefficient than a piezoelectric/electrostrictive material forming thepiezoelectric/electrostrictive element because thepiezoelectric/electrostrictive device excellent in temperaturecharacteristic can be obtained. For obtaining thepiezoelectric/electrostrictive device with better temperaturecharacteristic, it is preferable to use a material selected from thegroup consisting of Mo₂O₃, Nb₂O₅, U₃O₈, PbTiO₃, SrZrO₃, SiO₂, SiO₂ addedwith a trace amount of TiO₂, and cordierite.

The second piezoelectric/electrostrictive device comprises a pair ofmutually confronting thin plate portions, and a fixing portionsupporting the pair of thin plate portions, wherein movable portions areprovided at tip end portions of the pair of thin plate portions, themovable portions have mutually confronting end surfaces, and one or morepiezoelectric/electrostrictive elements are disposed on at least one ofthe pair of thin plate portions, and is characterized in that at leastboth side surfaces of the thin plate portions and one or morepiezoelectric/electrostrictive elements are covered with coating filmsformed using polysilazane. The coating film formed using polysilazane isconverted into a film formed of substantially SiO₂ only, and thusbecomes a film with a lower thermal expansion coefficient as comparedwith the piezoelectric/electrostrictive element.

In the production of the piezoelectric/electrostrictive device, when thepiezoelectric/electrostrictive element is formed on, for example, alater-described ceramic stacked body (obtained by stacking ceramic greensheets and firing them for integration), an internal residual stress isgenerated in the piezoelectric/electrostrictive element. Particularly,when the piezoelectric/electrostrictive element is formed into theceramic stacked body by firing for integration, the internal residualstress is liable to occur in the piezoelectric/electrostrictive elementdue to a difference in contraction and thermal expansion coefficient ofthe constituent members generated upon firing.

When the use of the piezoelectric/electrostrictive device is startedfrom this state, depending on the temperature of the ambient environmentor the element itself upon use, there are those instances where thedisplacement following the control does not occur in the movableportions when the predetermined electric field is applied topiezoelectric/electrostrictive layers forming thepiezoelectric/electrostrictive element.

A cause of this phenomenon can be considered such that due to adifference between a temperature upon production and a temperature uponuse, influence of the internal residual stress generated upon productionchanges. Specifically, the internal residual stress is large at roomtemperature so that the original displacing characteristic of thepiezoelectric/electrostrictive material is suppressed, while, as theusing temperature increases, the internal residual stress is lowered sothat the displacement of the movable portion becomes large.

In the first and second piezoelectric/electrostrictive devices accordingto the present invention, since at least both sides of the thin plateportions and the piezoelectric/electrostrictive elements are coveredwith the coating films of the low thermal expansion coefficientmaterial, a new stress is generated between the low thermal expansioncoefficient film and the piezoelectric/electrostrictive element as thetemperature increases to high, so as to suppress the increase ofexcessive displacement of the piezoelectric/electrostrictive element athigh temperatures. Therefore, even at high temperatures, it is possibleto obtain a displacing operation of the movable portion thatapproximates a design value and thus is highly accurate.

In the first and second piezoelectric/electrostrictive devices accordingto the present invention, it is preferable that a space is formedbetween the mutually confronting end surfaces of the movable portions.Since a portion of the movable portion including one of the end surfacesand another portion of the movable portion including the other endsurface, become liable to bend, the deformation resistance increases sothat handeability of the piezoelectric/electrostrictive device improves.Further, it is possible to achieve a reduction of the weight of themovable portions, and thus, it becomes possible to increase theresonance frequency without reducing the displacement amount of themovable portions. Accordingly, it is possible to achieve both a largedisplacement of the movable portions and a higher resonance frequency inthe displacing operation of the movable portions. For further reducingthe weight, it is preferable to shorten the overall length of themovable portions as represented by movable portions 42 shown in FIG. 22.

In the first and second piezoelectric/electrostrictive devices accordingto the present invention, like the previous proposal (SeeJP-A-2001-320103), the movable portions, the fixing portion, and thethin plate portions may be made of ceramic or metal. It is possible toform the respective portions of a ceramic material, or it is possible toform the respective portions of a metal material. Further, it is alsopossible to form a hybrid structure wherein the portions made of aceramic material and the portions made of a metal material are combinedtogether.

Normally, the pair of mutually confronting thin plate portions is madeof a material having a higher thermal expansion coefficient than thepiezoelectric/electrostrictive material forming thepiezoelectric/electrostrictive elements. For example, zirconia orstainless steel may be used.

More preferably, in the first and second piezoelectric/electrostrictivedevices according to the present invention, the thin plate portions, themovable portions, and the fixing portion are formed by a ceramic basebody obtained by simultaneously firing ceramic green laminates so as tobe integrated.

In the first and second piezoelectric/electrostrictive devices accordingto the present invention, it is preferable that thepiezoelectric/electrostrictive elements are integrated with the ceramicbase body through firing, and it is preferable that thepiezoelectric/electrostrictive element is in the form of a film, and hasa piezoelectric/electrostrictive layer and a pair of electrodes formedon the piezoelectric/electrostrictive layer. It is preferable that thepiezoelectric/electrostrictive element is formed by stacking a pluralityof the piezoelectric/electrostrictive layers and a plurality of pairs ofthe electrodes. With this arrangement, the generating power of thepiezoelectric/electrostrictive element is increased so that a largedisplacement can be achieved. Further, since the rigidity of the deviceitself is enhanced, a higher resonance frequency can be achieved so thata speed-up of the displacing operation can be easily accomplished.

It may also be arranged that one of the pair of electrodes is formed onat least the thin plate portion. This makes it possible that vibrationscaused by the piezoelectric/electrostrictive element are efficientlytransmitted to the movable portion via the thin plate portion, so thatthe response property can be improved.

The piezoelectric/electrostrictive device according to the presentinvention can be used as an active element such as a transducer, anactuator, a frequency region functioning component (filter), atransformer, a vibrator or a resonator for communication or power, anoscillator or a discriminator, or as a sensor element for varioussensors such as an ultrasonic sensor, an acceleration sensor, angularvelocity sensor, an impact sensor and a mass sensor. Particularly, itcan be suitably used for various actuators used in mechanisms foradjusting displacement, position and angle of various precisioncomponents of optical equipment, precision equipment etc.

Furthermore, according to the present invention, there are provided thefollowing two piezoelectric/electrostrictive elements.

The first piezoelectric/electrostrictive element is in the form of afilm and comprises a piezoelectric/electrostrictive layer and a pair ofelectrodes formed on the piezoelectric/electrostrictive layer, and ischaracterized in that at least a pair of side surfaces parallel to adisplacing direction are covered with coating films formed usingpolysilazane. The coating film formed using polysilazane becomes a filmformed of substantially SiO₂ only.

The second piezoelectric/electrostrictive element is in the form of afilm and comprises a piezoelectric/electrostrictive layer and a pair ofelectrodes formed on the piezoelectric/electrostrictive layer, and ischaracterized in that at least a pair of side surfaces parallel to adisplacing direction are covered with coating films made ofsubstantially SiO₂ only and each having a thickness of 0.1 μm orgreater.

The film made of only SiO₂ is a film having a lower thermal expansioncoefficient as compared with the piezoelectric/electrostrictive element.Specifically, in the first and second piezoelectric/electrostrictiveelements according to the present invention, at least a pair of sidesurfaces parallel to the displacing direction are covered with the filmshaving the lower thermal expansion coefficient as compared with thepiezoelectric/electrostrictive elements, so that, for example, as thetemperature increases to high following the driving of the elements, thelow thermal expansion coefficient film can suppress the temperaturecharacteristic induced by a difference in thermal expansion coefficientbetween the piezoelectric material of the piezoelectric/electrostrictiveelements and the electrode material. Therefore, even at hightemperatures, it is possible to manifest the displacing amount that ismore approximate to a design value and thus is highly accurate.

In the first and second piezoelectric/electrostrictive elementsaccording to the present invention, it is preferable that a plurality ofthe piezoelectric/electrostrictive layers are provided, and thepiezoelectric/electrostrictive layers and the electrodes are alternatelystacked so that the electrodes are provided on an uppermost surface anda lowermost surface, and it is preferable that thepiezoelectric/electrostrictive element is formed by stacking a pluralityof the piezoelectric/electrostrictive layers and a plurality of pairs ofthe electrodes. With this arrangement, generating power of thepiezoelectric/electrostrictive element is increased so that largedisplacement can be achieved. Further, since rigidity of thepiezoelectric/electrostrictive element is more enhanced, higherresonance frequency can be achieved so that the speed-up of thedisplacing operation can be accomplished.

Further, according to the present invention, there is provided the firstproduction method of producing a piezoelectric/electrostrictive devicecomprising a pair of mutually confronting thin plate portions, and afixing portion supporting the pair of thin plate portions, whereinmovable portions are provided at tip end portions of the pair of thinplate portions, the movable portions have mutually confronting endsurfaces, and one or more piezoelectric/electrostrictive elements aredisposed on at least one of the pair of thin plate portions, the methodcharacterized by comprising a step of, after forming the one or morepiezoelectric/electrostrictive elements on the at least one of the pairof thin plate portions, covering at least both side surfaces of the thinplate portions and the one or more piezoelectric/electrostrictiveelements with coating films made of a low thermal expansion coefficientmaterial by a film formation method. In this event, as the filmformation method, there can be used a method such as sticking of a filmyplate separately prepared in advance, coating, dipping, sputtering, CVD,or laser ablation.

Further, according to the present invention, there is provided a methodof producing a piezoelectric/electrostrictive element comprising apiezoelectric/electrostrictive layer and a pair of electrodes formed onthe piezoelectric/electrostrictive layer, the method comprising a stepof covering, by a film formation method, at least a pair of sidesurfaces parallel to a displacing direction with coating films made ofsubstantially SiO₂ only and each having a thickness of 0.1 μm orgreater. In this event, as the film formation method, it is preferableto adopt a coating method or a dipping method using a polysilazane.

Recently, as applied uses of the piezoelectric/electrostrictive elementand the piezoelectric/electrostrictive device, in addition to theconventional floor-type magnetic disk drive or optical disk drive, thereare increasing uses in those apparatuses that are susceptible tovibration or impact such as magnetic disk drives, optical disk drives,acceleration sensors, and angular velocity sensors for vehicle or mobileequipment, so that there are those instances where the mechanicalstrength is insufficient. The mechanical strength of the conventionalpiezoelectric/electrostrictive elements andpiezoelectric/electrostrictive devices is set to a value lower than avalue calculated from material values of the respective materialsforming the piezoelectric/electrostrictive device. The reason thereof isconsidered that, in addition to the fact that a stress remains in thepiezoelectric/electrostrictive layers and the thin plate portions(vibration plates) of the piezoelectric/electrostrictive elements due toa difference between a temperature upon production process and atemperature upon use, damages caused on thepiezoelectric/electrostrictive elements or the thin plate potions(vibration plates) during the production process of thepiezoelectric/electrostrictive device become notches, and the stress isconcentrated to notch portions.

In the aforementioned piezoelectric/electrostrictive device andpiezoelectric/electrostrictive element according to the presentinvention, the aforementioned notch portions are filled up by thecoating film to smooth the surface so as to relax the concentration ofthe stress, so that it is possible to improve the mechanical strength.Further, when the coating film of the low thermal expansion coefficientmaterial is formed at a temperature higher than the using temperature, acompressive stress remains in the coating film at the using temperatureso that cracks are not easily generated in the coating film, while, evenif the cracks are generated, development thereof is suppressed.Therefore, the mechanical strength can be improved resultantly.

The object of the present invention can also be accomplished bypiezoelectric/electrostrictive devices obtained by the followingproduction methods. The aforementioned first production method accordingto the present invention is means that can obtain the first and secondpiezoelectric/electrostrictive devices according to the presentinvention. On the other hand, the following second, third, and fourthproduction methods according to the present invention are not means forproducing the piezoelectric/electrostrictive device covered with thecoating films like the first and second piezoelectric/electrostrictivedevices according to the present invention, but means for obtaining apiezoelectric/electrostrictive device wherein joining between the mainconstituent members is implemented by the diffusion joining method.

The second production method according to the present invention is amethod of producing a piezoelectric/electrostrictive device comprising athin plate portion and a fixing portion supporting the thin plateportion and formed with a cavity inside, wherein one or morepiezoelectric/electrostrictive elements are disposed on the thin plateportion in a position corresponding to the cavity of the fixing portion,and is characterized by comprising the steps of preparing a joined bodyby joining a thin plate that becomes the thin plate portion later, and athick plate that comprises at least one layer and becomes the fixingportion later, through diffusion joining; and forming the one or morepiezoelectric/electrostrictive elements on the thin plate of the joinedbody.

The third production method according to the present invention is amethod of producing a piezoelectric/electrostrictive device comprising apair of mutually confronting thin plate portions and a fixing portionsupporting the pair of thin plate portions, wherein one or morepiezoelectric/electrostrictive elements are disposed on at least one ofthe pair of thin plate portions.

The third production method according to the present invention ischaracterized by comprising the steps of preparing a joined body byjoining thin plates that become the thin plate portions later, and oneor more thin plates or thick plates that become the fixing portionlater, through diffusion joining; disposing the one or morepiezoelectric/electrostrictive elements on at least one of the thinplates of the joined body to prepare an originalpiezoelectric/electrostrictive device; and cutting the originalpiezoelectric/electrostrictive device to obtain individualpiezoelectric/electrostrictive devices.

The fourth production method according to the present invention is amethod of producing a piezoelectric/electrostrictive device comprising apair of mutually confronting thin plate portions, and a fixing portionsupporting the pair of thin plate portions, wherein movable portions areprovided at tip end portions of the pair of thin plate portions, themovable portions have mutually confronting end surfaces, and one or morepiezoelectric/electrostrictive elements are disposed on at least one ofthe pair of thin plate portions.

The fourth production method according to the present invention ischaracterized by comprising the steps of preparing intermediate joinedbodies each by joining a thin plate that becomes the thin plate portionlater, and one or more thin plates or thick plates that become themovable portion and part of the fixing portion later, through diffusionjoining; preparing a joined body by joining the intermediate joinedbodies and one or more thin plates or thick plates that become thefixing portion later, through diffusion joining; disposing the one ormore piezoelectric/electrostrictive elements on at least one of the thinplates of the joined body to prepare an originalpiezoelectric/electrostrictive device; and cutting the originalpiezoelectric/electrostrictive device to obtain individualpiezoelectric/electrostrictive devices.

In the second to fourth production methods according to the presentinvention, it is preferable that the thin plate or thick plate thatbecomes at least part of the fixing portion later, is formed with awindow portion in advance. Of course, it may also be arranged that allthe plates including the thin plates that become the thin plate portionslater have window portions.

The window portions are hole portions opened in the thin plates and,although not forming the components of thepiezoelectric/electrostrictive device, they are necessary spaces fordetermining the shapes of the thin plate portions and the fixingportion, for example, that are the components of thepiezoelectric/electrostrictive device. Therefore, if no window portionsare provided, thin plates having final shapes or having shapes closelyapproximate to the final shapes are joined together. In this case,depending on the shapes or materials used for the thin plates, there ispossibility of deformation during the production process due toinsufficient strength. On the other hand, the thin plates having thewindow portions are in the form of frame-shaped members, so that it iseasy to ensure the strength and thus not easy to be deformed. The meansfor forming the window portions in the thin plates in advance ispreferable when, for example, the thin plates are ceramic green sheets.On the other hand, if the thin plates are metal plates, the windowportions may be formed in advance, or may not be formed, i.e. the metalplate with no window portions can also be used.

In the second to fourth production methods according to the presentinvention, all the thin plates and thick plates being the mainconstituent members are made of a material of the same kind forsuppressing change in shape before and after the diffusion joining.Since the thin plates and thick plates being the main constituentmembers of the piezoelectric/electrostrictive device are joined togetherby the diffusion joining method to obtain thepiezoelectric/electrostrictive device, joined portions are integratedwith the members themselves so that reliability of the joining is quitehigh. Since there exist no materials of different kinds in theconstituent members including the joined portions, a thermal stresscaused by change in temperature is suppressed to minimum so that thetemperature characteristic becomes excellent. Therefore, the producedpiezoelectric/electrostrictive device enables a small displacementcontrol with ultra-high accuracy even upon occurrence of change intemperature of the using environment or even upon use at hightemperatures. Further, since no adhesive agent layers exist at thejoined portions, the dimensional accuracy in the thickness direction ishigh.

In the second to fourth production methods according to the presentinvention, it is preferable that the thin plate or thick plate is madeof a material of which a 0.2% proof stress at 800° C. is 75 MPa orgreater. This is because deformation of the thin plate or thick plate(member to be joined) before and after the diffusion joining can besuppressed. FIG. 19 shows a relationship between the 0.2% proof stressat 800° C. of the typical metal material and the dimensional change(deformation) before and after the diffusion joining. As shown in FIG.19, as materials that satisfy the aforementioned condition, an 18Cr-8Nialloy (corresponding to SUS304), an 18Cr-8Nb alloy, and an 18Cr-8Moalloy can be cited.

In the diffusion joining method, a member to be joined (a thin plate orthick plate, or an intermediate joined body obtained by joining aplurality of thin plates) is placed between two pressure dies andpressed at a predetermined temperature. In this event, it is preferablethat the member to be joined is pressed in the state where pressureplates made of the same material as that of the member to be joined andapplied with solid lubricant are interposed between the pressure diesand the member to be joined. By using the pressure plates made of thesame material as that of the member to be joined, i.e. having the samethermal expansion coefficient, deformation of the member to be joinedthat is sandwiched between the pressure dies, can be prevented and, byapplying the solid lubricant to the pressure plates, joining between thepressure plates and the member to be joined can be prevented. It ispreferable that the solid lubricant contains at least hexagonal boronnitride. This is because, even at high temperature and under highpressure upon the diffusion joining, there occurs no reaction with oradhesion to the member to be joined.

In the aforementioned diffusion joining method, it is also preferablethat the member to be joined is pressed in the state where ceramicpressure plates having a thermal expansion coefficient that is within arange of ±30% relative to a thermal expansion coefficient of the memberto be joined, are interposed between the pressure dies and the member tobe joined. This is because deformation of the thin plate or thick plate(member to be joined) before and after the diffusion joining can besuppressed. FIG. 20 shows the ratio of thermal expansion coefficient ofthe pressure plate relative to thermal expansion coefficient of themember to be joined, and the dimensional change (deformation) thereofbefore and after the diffusion joining. As shown in FIG. 20, if thethermal expansion coefficient of the pressure plate relative to themember to be joined is 70% or greater, i.e. if it is a (ceramic)pressure plate having a thermal expansion coefficient that is within therange of 30% relative to a thermal expansion coefficient of the memberto be joined, the dimensional change can be suppressed to approximately1% or less. It is preferable that the ceramic pressure plates are madeof calcium oxide (CaO) or magnesium oxide (MgO) of 80% or more purity.If the purity is lower than it, the thermal expansion coefficientbecomes extremely low so that deformation of the member to be joinedbefore and after the diffusion joining becomes remarkable.

The aforementioned second, third, and fourth production methods of thepiezoelectric/electrostrictive devices according to the presentinvention are common in using the diffusion joining method for joiningthe main constituent members, and any of them can be used along with theaforementioned first production method. Specifically, if thepiezoelectric/electrostrictive devices obtained by the second, third,and fourth production methods are applied with the coating films of thelow thermal expansion coefficient material by the film formation methodsuch that at least both side surfaces of the thin plate portions and thepiezoelectric/electrostrictive elements are covered with the coatingfilms, the obtained piezoelectric/electrostrictive devices are eachprovided with highly excellent temperature characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an embodiment of apiezoelectric/electrostrictive device according to the presentinvention;

FIG. 2 is a perspective view showing the embodiment of thepiezoelectric/electrostrictive device according to the presentinvention, wherein a component is attached;

FIG. 3 is a perspective view showing an example of a conventionalpiezoelectric/electrostrictive device;

FIG. 4 is a perspective view showing another embodiment of apiezoelectric/electrostrictive device according to the presentinvention;

FIG. 5 is a perspective view showing still another embodiment of apiezoelectric/electrostrictive device according to the presentinvention;

FIGS. 6(a), (b) and (c) are plan views showing a step by step embodimentof a first production method for forming apiezoelectric/electrostrictive device according to the presentinvention;

FIG. 7 is a graph showing a result of a temperature characteristic testin an example;

FIGS. 8(a) and (b) are perspective views showing another embodiment ofthe first production method according to the present invention;

FIGS. 9(a) and (b) are perspective views showing still anotherembodiment of the first production method according to the presentinvention;

FIGS. 10(a) and (b) are perspective views showing still anotherembodiment of the first production method according to the presentinvention;

FIG. 11 is a perspective view showing an example of a conventionalpiezoelectric actuator;

FIG. 12 is a sectional view showing an application example of apiezoelectric/electrostrictive element according to the presentinvention;

FIGS. 13(a) and (b) are plan views showing an embodiment of a fourthproduction method of making a piezoelectric/electrostrictive deviceaccording to the present invention, which is a diagram for explaining aportion of the production processes;

FIGS. 14(a) and (b) are plan views showing the embodiment of the fourthproduction method of the piezoelectric/electrostrictive device accordingto the present invention, which is a diagram for explaining a portion ofthe production processes;

FIGS. 15(a) and (b) are plan views showing the embodiment of the fourthproduction method of the piezoelectric/electrostrictive device accordingto the present invention, which is a diagram for explaining a portion ofthe production processes;

FIG. 16 is a plan view showing the embodiment of the fourth productionmethod of the piezoelectric/electrostrictive device according to thepresent invention, which is a diagram for explaining a portion of theproduction processes;

FIG. 17 is a side view showing the embodiment of the fourth productionmethod of the piezoelectric/electrostrictive device according to thepresent invention, which is a diagram for explaining a diffusion joiningmethod;

FIGS. 18(a), (b) and (c) are perspective views showing an embodiment ofa second production method of making a piezoelectric/electrostrictivedevice according to the present invention, which is a diagram forexplaining the production processes;

FIG. 19 is a graph showing a relationship between the 0.2% proof stressat 800° C. of a thin plate or a thick plate used in the second, third orfourth production method of the piezoelectric/electrostrictive deviceaccording to the present invention, and the dimensional change thereofbefore and after diffusion joining;

FIG. 20 is a graph showing a relationship between the ratio of thermalexpansion coefficient of a ceramic pressure plate used in the second,third or fourth production method of the piezoelectric/electrostrictivedevice according to the present invention relative to thermal expansioncoefficient of a member to be joined, and the dimensional change thereofbefore and after diffusion joining;

FIGS. 21(a)-(g) are perspective views showing another embodiment of thefourth production method according to the present invention, whereinFIGS. 21(a) to 21(e) are diagrams for explaining the productionprocesses, FIG. 21(f) is a perspective view of a producedpiezoelectric/electrostrictive device, and

FIG. 21(g) is a side view of the produced piezoelectric/electrostrictivedevice; and

FIG. 22 is a perspective view showing still another embodiment of apiezoelectric/electrostrictive device according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the accompanying drawings, the numerical references have the meaningsdescribed below:

-   -   10, 80, 100, 140, 150, 300: piezoelectric/electrostrictive        device;    -   12 a, 12 b, 142, 312, 412: thin plate portion;    -   14,314,414: fixing portion;    -   16: substrate;    -   18 a, 18 b, 78, 88, 178, 378: piezoelectric/electrostrictive        element;    -   19 a, 19 b, 132: actuator portion;    -   20 a, 20 b, 320, 420: movable portion;    -   22: piezoelectric/electrostrictive layer;    -   24, 26: pair of electrodes;    -   34 a, 34 b, 334: end surface;    -   36: space;    -   41, 42, 43, 141, 341, 342, 343: window portion;    -   44: adhesive agent applying portion;    -   61, 62: thick plate;    -   63: cavity;    -   64: liquid draining hole;    -   65: rubber band;    -   69, 369: cutting line;    -   71, 72, 74, 171, 371, 372, 374: thin plate;    -   73 a, 73 b, 75: preliminary stacked body;    -   76, 174, 376: joined body;    -   77, 377: original piezoelectric/electrostrictive device;    -   79 a, 79 b, 373 a, 373 b: intermediate joined body;    -   81: filmy plate;    -   82: vibration plate;    -   91, 92, 101, 141, 151: coating film;    -   124: display element;    -   126: light source;    -   128: light;    -   130: optical waveguide plate;    -   134: driving portion;    -   140 a: pixel forming member;    -   142: through hole;    -   161: pressure chamber;    -   162: liquid introducing port;    -   163: liquid discharging port;    -   170: droplet discharging device;    -   172, 173: thick plate;    -   181: pressure die;    -   182: pressure plate;    -   200: plate-like member;    -   202: hole portion;    -   204: fixing portion;    -   206: movable portion;    -   208: beam portion;    -   210: electrode layer;    -   221: magnetic head; and    -   222, 223: joining portion.

Embodiments of piezoelectric/electrostrictive devices,piezoelectric/electrostrictive elements, and methods for productionthereof according to the present invention will be described hereinbelowwith reference to examples shown in the accompanying drawings. However,the present invention should not be interpreted as being limitedthereto, but can be added with various changes, alterations, andimprovements based on knowledge of experts in the art without departingfrom the scope of the present invention.

In the following description, the aforementioned first and secondpiezoelectric/electrostrictive devices will be collectively referred tosimply as the piezoelectric/electrostrictive device according to thepresent invention, and the aforementioned first and secondpiezoelectric/electrostrictive elements will be collectively referred tosimply as the piezoelectric/electrostrictive element according to thepresent invention. Further, the piezoelectric/electrostrictive elementaccording to the present invention can be a component of thepiezoelectric/electrostrictive device according to the presentinvention.

A coating film made of polysilazane exhibits a peculiar effect asdescribed later. In this specification, a coating film made of amaterial with a low thermal expansion coefficient includes the coatingfilm made of polysilazane.

Further, in this specification, a piezoelectric/electrostrictive devicerepresents a device that performs mutual conversion between electricalenergy and mechanical energy by means of apiezoelectric/electrostrictive element. Therefore, thepiezoelectric/electrostrictive device according to the present inventionis most preferably used as an active element such as each of variousactuators or vibrators, particularly as a displacement control elementutilizing the displacement caused by an inverse piezoelectric effect oran electrostrictive effect and, in addition, preferably used as apassive element such as an acceleration sensor element or an impactsensor element.

The piezoelectric/electrostrictive device according to the presentinvention shown in the following embodiments is apiezoelectric/electrostrictive device that is obtained by applying acoating film with a low thermal expansion coefficient to apiezoelectric/electrostrictive device 10 shown in FIG. 3 and disclosedin JP-A-2001-320103, and is thus improved in temperature characteristicto manifest excellent displacement relative to a given electric fieldeven at high temperatures.

First, this piezoelectric/electrostrictive device 10 will be described.

As shown in FIG. 3, the piezoelectric/electrostrictive device 10comprises a base body 16 including a pair of mutually confronting thinplate portions 12 a and 12 b and a fixing portion 14 supporting thesethin plate portions 12 a and 12 b that are formed integral with eachother, and further comprises piezoelectric/electrostrictive elements 18a and 18 b formed on portions of the pair of thin plate portions 12 aand 12 b, respectively.

The piezoelectric/electrostrictive device 10 is configured such that thepair of thin plate portions 12 a and 12 b are displaced by driving thepiezoelectric/electrostrictive elements/element 18 a and/or 18 b, or thedisplacement of the thin plate portions 12 a and 12 b is detected by thepiezoelectric/electrostrictive elements/element 18 a and/or 18 b.Namely, the thin plate portions 12 a and 12 b and thepiezoelectric/electrostrictive elements 18 a and 18 b form actuatorportions 19 a and 19 b, respectively.

Further, the tip end portions of each of the pair of thin plate portions12 a and 12 b increase in thickness in an inward direction, and thesethickness-increased portions serve as movable portions 20 a and 20 bthat are displaced following the displacing operations of the thin plateportions 12 a and 12 b, respectively. Hereinafter, the tip end portionsof the pair of thin plate portions 12 a and 12 b will be referred to asthe movable portions 20 a and 20 b, respectively. A space 36 isinterposed between mutually confronting end surfaces 34 a and 34 b ofthe movable portions 20 a and 20 b.

The base body 16 may be formed of ceramic or metal entirely, or may havea hybrid structure in combination of a member formed of ceramic and amember formed of metal. The base body 16 may adopt a structure whereinrespective portions are joined together by a joining agent such asorganic resin or glass, a ceramic integral structure wherein ceramicgreen laminates are integrated by firing, a metal integral structurewherein respective portions are integrated by diffusion joining,brazing, soldering, eutectic joining, welding, or the like. Inasmuch assubstantially no time-domain change occurs in the state, so that thereliability of joined portions is high, and that there is an advantagein ensuring rigidity, the base body 16 is preferably formed by a ceramicstacked body obtained by integrating the ceramic green laminates throughfiring.

The piezoelectric/electrostrictive elements 18 a and 18 b may beprepared as separate members and attached to the base body 16 by ajoining agent such as organic resin or glass, brazing, soldering,eutectic joining, or the like. Alternatively, thepiezoelectric/electrostrictive elements 18 a and 18 b may be formeddirectly on the base body 16 using the film forming method, not in theform of the attachment. More preferably, the base body 16 is formed asthe ceramic stacked body, and the piezoelectric/electrostrictiveelements 18 a and 18 b are integrated with the base body 16 by firing.

Now, the piezoelectric/electrostrictive device according to the presentinvention will be described.

The piezoelectric/electrostrictive device according to the presentinvention is obtained by covering at least both side surfaces of thethin plate portions and the piezoelectric/electrostrictive elements ofthe aforementioned piezoelectric/electrostrictive device 10 with coatingfilms made of a material with a low thermal expansion coefficient. FIG.1 shows one embodiment thereof. In a piezoelectric/electrostrictivedevice 100 according to the present invention, only both side surfacesof thin plate portions 12 a and 12 b and piezoelectric/electrostrictiveelements 18 a and 18 b are covered with coating films 101 (hatchedportions in the figure). Although a production method thereof will bedescribed later, the coating films of the piezoelectric/electrostrictivedevice 100 can be formed by, for example, masking those portions otherthan such portions where the coating films are formed in thepiezoelectric/electrostrictive device 10, i.e. other than both sidesurfaces of the thin plate portions 12 a and 12 b and thepiezoelectric/electrostrictive elements 18 a and 18 b, and byimplementing the film forming method such as sputtering, CVD or laserablation (implementing twice because of both surfaces).

Each of the piezoelectric/electrostrictive elements 18 a and 18 bcomprises filmy piezoelectric/electrostrictive layers 22 composed offour layers, and a pair of electrodes 24 and 26 formed on both surfacesof each piezoelectric/electrostrictive layer 22, and the electrodes 26of the pairs of electrodes 24 and 26 are formed on the pair of thinplate portions 12 a and 12 b (i.e. on the lowermost surfaces) and on theuppermost surfaces of the piezoelectric/electrostrictive elements 18 aand 18 b.

In the piezoelectric/electrostrictive device 100, when, for example, avoltage is applied to the pairs of electrodes 24 and 26 of thepiezoelectric/electrostrictive element 18 a, thepiezoelectric/electrostrictive layers 22 of thepiezoelectric/electrostrictive element 18 a are displaced by contractionin the principal plane direction thereof. As shown in FIG. 1, thiscauses the occurrence of a stress relative to the thin plate portion 12a in a bending direction (direction identified by arrow A) of the thinplate portion 12 a, so that the thin plate portion 12 a is bent in thedirection identified by the arrow A. In this event, assuming that themovable portions 20 a and 20 b are coupled together via a magnetic head221 interposed therebetween as shown in FIG. 2, and that the voltage isnot applied to the pairs of electrodes 24 and 26 of the otherpiezoelectric/electrostrictive element 18 b, the other thin plateportion 12 b is also bent in the direction identified by the arrow Afollowing the bend of the thin plate portion 12 a. As a result, themovable portions 20 a and 20 b are displaced in the direction identifiedby the arrow A relative to a longitudinal axis of thepiezoelectric/electrostrictive device 100.

As described above, the small displacement of thepiezoelectric/electrostrictive elements 18 a and 18 b is amplified asthe large displacing operation utilizing the bend of the thin plateportions 12 a and 12 b so as to be transferred to the movable portions20 a and 20 b, so that the movable portions 20 a and 20 b can be largelydisplaced relative to the longitudinal axis of thepiezoelectric/electrostrictive device 10.

Particularly, inasmuch as the space 36 is defined between the movableportions 20 a and 20 b, the weight reduction is further achieved so thatthe resonance frequency can be increased without reducing thedisplacement magnitude of the movable portions 20 a and 20 b. Thefrequency represents a frequency of voltage waveform when the voltageapplied to the pairs of electrodes 24 and 26 is alternately switched todisplace the movable portions 20 a and 20 b rightward and leftward,while the resonance frequency represents the maximum frequency at whichthe displacing operation of the movable portions 20 a and 20 b canfollow in a predetermined vibration mode.

The displacement magnitude normally changes depending on a value ofvoltage applied to (or electric field given to) thepiezoelectric/electrostrictive element. However, in the conventionalpiezoelectric/electrostrictive device, there have been those instanceswhere the manifested displacement did not agree with a control at themovable portions due to the fact that an influence of internal residualstress generated upon production changes due to a difference intemperatures upon production and use. Specifically, the displacingoperation of the movable portions sometimes became greater than acontrol value upon use at high temperatures.

In the piezoelectric/electrostrictive device 100 according to thepresent invention, inasmuch as both side surfaces of the thin plateportions 12 a and 12 b and the piezoelectric/electrostrictive elements18 a and 18 b, i.e. the pair of side surfaces parallel to the displacingdirection, are covered with the coating films 101 made of the materialhaving the lower thermal expansion coefficient as compared with thepiezoelectric/electrostrictive elements, the coating films 101 cansuppress the excessive displacement of thepiezoelectric/electrostrictive elements that is generated in thedirection identified by the arrow A as the temperature increases.Therefore, the displacement magnitude of the movable portions can becontrolled to a desired value even at the high temperatures, thereby toenable the piezoelectric/electrostrictive device 100 to operateaccurately.

Inasmuch as the width of the thin plate portions 12 a and 12 b isbasically constant even by increasing the driving force of the actuatorportions 19 a and 19 b, the piezoelectric/electrostrictive device 100 isa highly preferable device when applied to, for example, an actuator forcontrolling a position of an optical disk pickup or a hard disk magnetichead that is used in a very narrow gap.

FIG. 2 shows the state wherein the hard disk magnetic head is attachedto the piezoelectric/electrostrictive device 100 shown in FIG. 1. Themagnetic head 221 is fixed in the space 36 by joining portions 222 (endsurfaces 34 a and 34 b) of the movable portions 20 a and 20 b, while thepiezoelectric/electrostrictive device 100 itself attached with themagnetic head 221 is fixed to a hard disk suspension at a joiningportion 223. The joining portions 222 of the movable portions 20 a and20 b are the mutually confronting end surfaces 34 a and 34 b each havinga large surface area, so that the mountability of the magnetic head 221onto the movable portions 20 a and 20 b is improved to enable themagnetic head 221 to be fixed securely. In a hard disk drive,positioning of the piezoelectric/electrostrictive device 100 is firstcarried out by a voice coil motor (VCM) or the like, then positioning ofthe magnetic head 221 is accurately carried out by the movable portions20 a and 20 b that are displaced following the displacing operation ofthe piezoelectric/electrostrictive elements 18 a and 18 b.

Referring now to FIGS. 4 and 5, other embodiments of thepiezoelectric/electrostrictive device according to the present inventionwill be described.

In a piezoelectric/electrostrictive device 140 shown in FIG. 4, bothside surfaces of thin plate portions 12 a and 12 b including movableportions 20 a and 20 b, piezoelectric/electrostrictive elements 18 a and18 b, and a fixing portion 14, i.e. all the side surfaces, are coveredwith coating films 141 (hatched portions in the figure) made of amaterial with a low thermal expansion coefficient. End surfaces are notformed with the coating films 141. The coating films of thepiezoelectric/electrostrictive device 141 can be formed by, for example,implementing the film forming method such as sputtering, CVD or laserablation (implementing twice because of both surfaces) like thepiezoelectric/electrostrictive device 100.

In a piezoelectric/electrostrictive device 150 shown in FIG. 5, all thesurfaces (end surfaces and side surfaces) of thin plate portions 12 aand 12 b, including movable portions 20 a and 20 b,piezoelectric/electrostrictive elements 18 a and 18 b, and fixingportion 14, are covered with coating films 151 (hatched portions in thefigure) made of a material with a low thermal expansion coefficient. Thecoating films of the piezoelectric/electrostrictive device 150 can beeasily formed by, for example, the dipping method or the coating method.

The coating film of the present invention serves not only as a film forsuppressing the temperature characteristic, but also as a dampproof filmfor suppressing short circuit of a piezoelectric/electrostrictiveelement due to migration and corrosion of a metal base body and a metalthin plate portion at high temperature and high humidity, and breakagecaused by phase transformation of a partially stabilized zirconia basebody and thin plate portion, and as a dustproof film for suppressinggeneration of dust from a piezoelectric/electrostrictive device, whichwill be described later. Accordingly, it is preferable that moreportions of the piezoelectric/electrostrictive device are coated. Inview of this, the mode of the piezoelectric/electrostrictive device 140is preferable to the mode of the piezoelectric/electrostrictive device100, and further, the mode of the piezoelectric/electrostrictive device150 is more preferable.

In each of the piezoelectric/electrostrictive devices 140 and 150, likein the piezoelectric/electrostrictive device 100, inasmuch as both sidesurfaces of the thin plate portions 12 a and 12 b and thepiezoelectric/electrostrictive elements 18 a and 18 b, i.e. the pair ofside surfaces parallel to the displacing direction, are covered with thecoating films made of the material having the lower thermal expansioncoefficient as compared with the piezoelectric/electrostrictiveelements, the coating films suppress the excessive displacement of thepiezoelectric/electrostrictive elements that is generated as thetemperature increases, so that the piezoelectric/electrostrictive devicefollows the given electric field to manifest the desired displacementmagnitude accurately even at high temperatures. Namely, thepiezoelectric/electrostrictive device according to the present inventioncan achieve the certain effect as long as at least both side surfaces ofthe thin plate portions and the piezoelectric/electrostrictive elementsare covered with the coating films made of the material having the lowerthermal expansion coefficient as compared with thepiezoelectric/electrostrictive elements.

In the piezoelectric/electrostrictive device 100, 140, 150 according tothe present invention, it is sufficient that the material of the coatingfilms has the lower thermal expansion coefficient as compared with thepiezoelectric/electrostrictive elements as described above. On the otherhand, for further improving the temperature characteristic, it ispreferable to use one of materials like Mo₂O₃, Nb₂O₅, U₃O₈, PbTiO₃,SrZrO₃, SiO₂, SiO₂ added with a trace amount of TiO₂, and cordierite.These materials have thermal expansion coefficients of about 0.05 to1.0×10⁻⁶/° C., and are excellent in adhesion to apiezoelectric/electrostrictive material or an electrode material so thata coating film can be easily formed thereon.

Here, description will be given to effects that are achieved by thepiezoelectric/electrostrictive device according to the presentinvention.

In the present invention, as described above, the first effect is thatthe temperature characteristic of the piezoelectric/electrostrictivedevice becomes excellent. Apart from it, according to the mode of thepresent invention, i.e. being the piezoelectric/electrostrictive devicein which at least both side surfaces of the thin plate portions and thepiezoelectric/electrostrictive elements are covered with the coatingfilms made of the material having the lower thermal expansioncoefficient as compared with the piezoelectric/electrostrictiveelements, the following secondary effects are manifested.

The second effect is to prevent generation of particles. In thepiezoelectric/electrostrictive device according to the presentinvention, since both side surfaces of thepiezoelectric/electrostrictive elements are covered with the coatingfilms, generation of particles at least from the side surfaces of thepiezoelectric/electrostrictive elements can be suppressed, so that it ispossible to reduce the generation of particles over a long term. Themore preferable mode for reducing the generation of particles is the oneshown in FIG. 5, wherein the entire piezoelectric/electrostrictivedevice, including the piezoelectric/electrostrictive elements, iscovered with the coating films.

In general, when using a piezoelectric/electrostrictive element, sincethe piezoelectric/electrostrictive material itself is fragile, theprobability is high that the piezoelectric/electrostrictive elementitself is subjected to the occurrence of breakage or cracks. Therefore,particularly, if it is operated over a long term, the grain boundariesof crystals are exfoliated so that particles tend to be generated. Nopiezoelectric/electrostrictive materials have been found that areimproved to substantially prevent long-term particle generation.Therefore, there is possibility that the aforementioned problem aboutthe piezoelectric/electrostrictive element will directly lead to aserious problem depending on use thereof.

For example, when used for positioning a hard disk magnetic head asdescribed above, generated particles may make the disk and/or headdirty, which may not only cause an error in the reading/writingoperation, but may also induce breakage of an apparatus. If thepiezoelectric/electrostrictive device according to the present inventionis used, no such problem is raised.

The third effect is to improve durability of thepiezoelectric/electrostrictive device. In thepiezoelectric/electrostrictive device according to the presentinvention, inasmuch as both side surfaces of thepiezoelectric/electrostrictive elements are covered with the coatingfilms, even if the piezoelectric/electrostrictive device is usedparticularly in a high humidity atmosphere, invasion of moisture issuppressed so as to reduce the rate of occurrence of short circuitcaused by migration or the like over a long term, so that highreliability can be obtained. The more preferable mode for improving thedurability is the one shown in FIG. 5, wherein the entirepiezoelectric/electrostrictive device is covered with the coating films.

Particularly, when the coating films are made of polysilazane, since, asdescribed later, polysilazane chemically changes into a silica (SiO₂)film while consuming moisture, the moisture in the high humidityatmosphere, as well as moisture existing in thepiezoelectric/electrostrictive elements or thepiezoelectric/electrostrictive device is removed. Accordingly, theinside of the coating films is always in a dry state so that it becomesmore difficult to induce deterioration.

The fourth effect is to prevent adhesive failure of components etc. Inthe piezoelectric/electrostrictive device according to the presentinvention, the adhesive property of the surfaces (side surfaces and endsurfaces) of the piezoelectric/electrostrictive device can be improvedby covering the whole piezoelectric/electrostrictive device with thecoating films as shown in FIG. 5.

For example, when using the piezoelectric/electrostrictive deviceaccording to the present invention for positioning the hard diskmagnetic head as described above, the magnetic head is joined to the endsurfaces of the movable portions as shown in FIG. 2, or thepiezoelectric/electrostrictive device itself is joined to the hard disksuspension or the like. On the other hand, conventionally, since theadhesion property of the surfaces of the piezoelectric/electrostrictivedevice is not good, sufficient adhesive strength cannot be obtained.

The reason why the adhesive property of the surfaces of thepiezoelectric/electrostrictive device is not good is considered asfollows: Upon processing a piezoelectric/electrostrictive device into arequired shape, the processing such as wire sawing or dicing is carriedout. Since the piezoelectric/electrostrictive device is very small (e.g.about 1 to 2 mm between the thin plate portions, about 0.05 to 0.5 mm inthickness (width of end surface)), it is difficult to completely removechips or abrasive grains adhered to the processing surfaces so that thejoining is performed via those residual chips or abrasive grains.

When the piezoelectric/electrostrictive device according to the presentinvention is used, inasmuch as the coating films are formed after theprocessing, no such a problem is raised. A resin film is inferior inadhesive property and thus is not preferable. An inorganic film ispreferable for the coating film, which is preferably made of theaforementioned low thermal expansion coefficient material like Mo₂O₃,Nb₂O₅, U₃O₈, PbTiO₃, SrZrO₃, SiO₂, SiO₂ added with a trace amount ofTiO₂, or cordierite.

Hereinbelow, an embodiment of the piezoelectric/electrostrictive elementaccording to the present invention will be described with an applicationexample given.

The piezoelectric/electrostrictive element according to the presentinvention is a filmy piezoelectric/electrostrictive element having anpiezoelectric/electrostrictive layer and a pair of electrodes formed onthe piezoelectric/electrostrictive layer, wherein at least a pair ofside surfaces parallel to the displacing direction are covered withcoating films. Each of the coating films is a film formed ofpolysilazane, or a film formed of substantially SiO₂ only and having athickness of 0.1 μm or greater.

Irrespective of whether each of the coating films covering at least thepair of side surfaces parallel to the displacing direction is the filmformed of polysilazane, or the film formed of substantially SiO₂ onlyand having the thickness of 0.1 μm or greater, it is a film having alower thermal expansion coefficient than apiezoelectric/electrostrictive material forming thepiezoelectric/electrostrictive element, so that thepiezoelectric/electrostrictive element according to the presentinvention can be preferably applied to thepiezoelectric/electrostrictive device according to the present inventionthat has been already described.

FIG. 12 is a sectional view of a display element for a display devicebeing another application example of the piezoelectric/electrostrictiveelement according to the present invention. A display element 124comprises an optical waveguide plate 130 into which light 128 from alight source 126 is introduced, and a driving portion 134 provided so asto confront the back of the optical waveguide plate 130 and having manyactuator portions 132 arranged in a matrix or zigzag fashioncorrespondingly to pixels.

Although a pixel array configuration is not shown, for example, twoactuator portions 132 arrayed vertically form one dot, and three dots(red dot, green dot, and blue dot) are arrayed horizontally to form onepixel. In the display element 124, pixel forming members 140 a arestacked in layers on each actuator portion 132, and the pixel formingmembers 140 a are displaced upward and downward (in the figure)following displacement of each actuator portion 132 to increase acontact area with the optical waveguide plate 130, thereby to achievethe area corresponding to a pixel so as to express a color image.

Normally, in case of such a display element for the display device, whenoperation is started, it continues over a long term, and thetemperature, humidity, and so on of ambient environment where it is usedare not necessarily good conditions. Therefore, higher durability isrequired for the respective components. Thepiezoelectric/electrostrictive element according to the presentinvention is provided with, among the aforementioned first to fourtheffects of the piezoelectric/electrostrictive device according to thepresent invention, the second and third effects relating to thepiezoelectric/electrostrictive element. In other words, it is adisplacement control element that is resistant to occurrence ofparticles and excellent in durability, and thus is suitable as theactuator portion 132 of the display element 124. Particularly, when acoating film covering the actuator portion 132 and a thin plate portion142 is made of polysilazane, the inside of the coating film is always ina dry state so that it is possible to fully avoid adverse influencecaused by the ambient high humidity, or deterioration caused byinternally existing moisture.

Now, the materials for forming the piezoelectric/electrostrictive deviceand the piezoelectric/electrostrictive element according to the presentinvention will be described.

As a material forming the movable portions and the fixing portion of thepiezoelectric/electrostrictive device, there is no particular limitationas long as it has rigidity. On the other hand, ceramics to which thelater-described ceramic green sheet stacking method is applicable can bepreferably used. Specifically, there can be cited those materials eachcontaining, as a main component, zirconia such as stabilized zirconia orpartially stabilized zirconia, alumina, magnesia, silicon nitride,aluminum nitride, or titanium oxide, and further, those materials eachcontaining a mixture thereof as a main component. However, in view ofhigh mechanical strength or toughness, the material containing zirconia,particularly, stabilized zirconia or partially stabilized zirconia, asthe main component is preferable. With respect to metal materials, thereis no limitation as long as they have rigidity. However, there can becited stainless steel, nickel, spring steel, brass, beryllium copper,and so on.

As a material forming the thin plate portions, the same ceramics for themovable portions and the fixing portion can be preferably used. Amongthem, zirconia, particularly a material containing stabilized zirconiaas a major component and a material containing partially stabilizedzirconia as a major component are preferably usable because themechanical strength is large and the toughness is high even in case of asmall thickness, and reactivity with the piezoelectric/electrostrictivelayers and the electrode material is small. When forming the thin plateportions with a metal material, it is sufficient that the metal materialhas flexibility and is bendable to deform. However, as ferrousmaterials, various stainless steel products and various spring steelproducts are preferable, while, as non-ferrous materials, brass,beryllium copper, phosphor bronze, nickel, and a nickel-iron alloy arepreferable.

In the piezoelectric/electrostrictive element, piezoelectric ceramicsare preferably used for the piezoelectric/electrostrictive layers, butit is also possible to use electrostrictive ceramics, ferroelectricceramics, or antiferroelectric ceramics. As concrete materials, therecan be cited those ceramics each containing lead zirconate, leadtitanate, lead magnesium niobate, lead nickel niobate, lead zincniobate, lead manganese niobate, lead antimony stannate, lead manganesetungstate, lead cobalt niobate, barium titanate, sodium bismuthtitanate, bismuth neodymium titanate, potassium sodium niobate,strontium bismuth tantalate, or the like alone or as a mixture thereof.

It is preferable that the electrode of thepiezoelectric/electrostrictive element is made of a metal that is asolid body at room temperature with excellent conductivity. For example,aluminum, titanium, chrome, iron, cobalt, nickel, copper, zinc, niobium,molybdenum, ruthenium, palladium, rhodium, silver, tin, tantalum,tungsten, iridium, platinum, gold, or lead are used alone or as an alloythereof. Further, a cermet material obtained by dispersing the samematerial as that of the piezoelectric/electrostrictive layer or the thinplate portion into such metals may also be used.

Now, the first production method including a process of applying thecoating films of the piezoelectric/electrostrictive device according tothe present invention will be described with reference to the figures.Description about a production method of thepiezoelectric/electrostrictive element according to the presentinvention will also be included herein.

In the piezoelectric/electrostrictive device according to the presentinvention, constituent materials of the respective members are ceramics,and it is preferable to produce the base body excluding thepiezoelectric/electrostrictive elements, i.e. the thin plate portions,the fixing portion, and the movable portions, using the ceramic greensheet stacking method described hereinbelow. The reason therefor is thatthere occurs substantially no time-domain change in state at joinedportions of the respective members so that reliability of the joinedportions is high, and there is an advantage in ensuring rigidity. On theother hand, with respect to the piezoelectric/electrostrictive elements,electrode terminals, and the like, it is preferable to produce themusing the thin or thick film formation method. The production methodsbased on these means are excellent in productivity and formability andcan obtain the piezoelectric/electrostrictive devices with highreproducibility in a short time.

First, the ceramic green sheet stacking method will be described. Abinder, a solvent, a dispersing agent, a plasticizer, etc. are added toceramic powder such as zirconia powder, which are mixed to produceslurry. After degassing the slurry, a ceramic green sheet having apredetermined thickness is produced by the reverse roll coater method,the doctor blade method, etc. Then, using the method such as thepunching processing using dies or the laser processing, the ceramicgreen sheet is processed into a predetermined shape to obtain aplurality of ceramic green sheets for forming a base body. Thereafter,the ceramic green sheets are stacked and press-joined to be formed intoa ceramic green stacked body, which is then fired to obtain a ceramicstacked body.

Then, the piezoelectric/electrostrictive elements are formed on bothsurfaces of the ceramic stacked body without using a joining agent byusing, for example, a thick film formation method such as a screenprinting method, a dipping method, a coating method or anelectrophoretic method, or by a thin film formation method such as anion beam method, a sputtering method, a vacuum evaporation method, anion plating method, a chemical vapor deposition (CVD) method or aplating method. Thick film formation method is preferred.

Details about the process for preparing the base body, forming thepiezoelectric/electrostrictive elements, and making up the shape of thepiezoelectric/electrostrictive device follow the description ofJP-A-2001-320103. As described therein, a plurality of productionprocesses are implemented.

Subsequently, at least both side surfaces of the thin plate portions andthe piezoelectric/electrostrictive elements are covered with coatingfilms made of a material with a low thermal expansion coefficient usingthe film formation method. It is also preferable to cover the whole sidesurfaces of the piezoelectric/electrostrictive device including themovable portions and the fixing portion in addition to the thin plateportions and the piezoelectric/electrostrictive elements, with thecoating films of the low thermal expansion coefficient material.Further, it may also be arranged that the wholepiezoelectric/electrostrictive device including the end surfaces iscovered with the coating films of the low thermal expansion coefficientmaterial. Herein, both side surfaces of thepiezoelectric/electrostrictive elements represent such surfaces that areparallel to the displacing direction.

There is no limitation about the low thermal expansion coefficientmaterial to be used as long as it has a thermal expansion coefficientlower than that of a piezoelectric/electrostrictive material forming thepiezoelectric/electrostrictive elements. For example, Mo₂O₃, Nb₂O₅,U₃O₈, PbTiO₃, SrZrO₃, SiO₂, SiO₂ added with a trace amount of TiO₂, orcordierite may be used. Among them, it is preferable to form the coatingfilm of substantially silica (SiO₂) only.

As the film formation method used in the formation of the coating film,means such as sticking a separately prepared filmy plate, coating,dipping, sputtering, CVD, or laser ablation can be adopted. Taking intoconsideration the low thermal expansion coefficient material to be used,and the portion and area where the coating film is formed, a suitablemethod that is easy to apply may be used.

FIGS. 6(a) and 6(c) are plan views for explaining processes of formingthe coating films using the dipping method, wherein the whole of thepiezoelectric/electrostrictive device 10 (see FIG. 3) is covered withthe coating films of the low thermal expansion coefficient material toproduce the piezoelectric/electrostrictive 150 (see FIG. 5). Here, thecoating films are formed of silica (SiO₂).

First, a thick plate 61 (e.g. made of PTFE) having many small dippingbaths for dipping therein piezoelectric/electrostrictive devices 10, isprepared. The thick plate 61 is formed with many cavities 63 each havinga liquid draining hole 64 and having a shape that agrees with a shape ofthe piezoelectric/electrostrictive device 10, and each cavity 63 servesas a dipping bath. Then, the piezoelectric/electrostrictive devices 10are placed in the cavities 63, and a thick plate 62 having the sameshape as the thick plate 61 is reversed to cover the thick plate 61.Then, the thick plate 61 and the thick plate 62 are fixed together usingrubber bands 65 having solvent resistance, or the like, so as to preventthe thick plate 62 from being detached from the thick plate 61.

Then, the thick plates 61 and 62 with the piezoelectric/electrostrictivedevices 10 accommodated therein are dipped into a polysilazane solutionthat has been diluted to, for example, 20 mass % by xylene. After takingout the piezoelectric/electrostrictive devices 10 from the polysilazanesolution, the excessive solution is removed by, for example, blowingnitrogen gas to dry them, and further, xylene is removed by heating todry them, for example, at 120° C. for 30 minutes. Thereafter, a heattreatment is applied to them, for example, at 450° C. for about 2 hours.

Through the aforementioned processes, films of polysilazane adhered toall the surfaces of each piezoelectric/electrostrictive device 10 bydipping are converted into ceramic fine coating films made ofsubstantially silica only due to oxidation or hydrolysis, so that thepiezoelectric/electrostrictive device 150 covered with the coating filmsentirely as shown in FIG. 5 can be obtained.

Polysilazane (—SiH₂NH—) has a width in average molecular weight over arange of about 300 to 5000. There also exists polysilazane containing anoxidation catalyst or a dehydrogenation agent. Any of such polysilazaneswill do when used for forming the coating films on thepiezoelectric/electrostrictive device or thepiezoelectric/electrostrictive element according to the presentinvention. However, since it is possible that viscosity changesdepending on molecular weight, it is preferable to use polysilazanethrough dilution to a suitable concentration, not limited to theaforementioned example, by xylene or the like for controlling thethickness of films adhered to the device by dipping to, preferably 0.1μm or greater. Further, it is preferable to properly change theaforementioned heating/drying time, heat treatment temperature, andrequired time therefor depending on the kind of polysilazane.

FIGS. 8 to 10 are perspective views for explaining processes of formingcoating films of a low thermal expansion coefficient material relativeto a piezoelectric/electrostrictive device 80 of a unimorph type havinga vibration plate 82 made of zirconia, and apiezoelectric/electrostrictive element 88 of a stacked type formedthereon.

FIGS. 8(a) and (b) show the state wherein separately prepared filmyplates 81 of a low thermal expansion coefficient material are stuck toside surfaces of the piezoelectric/electrostrictive device 80. Thismethod is applicable to a piezoelectric/electrostrictive device of arelatively large size. For each filmy plate 81, various kinds of glasshaving silica as a main component (e.g. soda glass) can be used. Thesticking may be implemented using an epoxy, urethane, or acrylicadhesive agent, or the like.

With respect to a piezoelectric/electrostrictive device of a relativelysmall size, it is preferable to form coating films directly on sidesurfaces of a piezoelectric/electrostrictive device 80 using a lowthermal expansion coefficient material as shown in FIGS. 9(a) and (b)and FIGS. 10(a) and (b). FIGS. 9(a) and (b) show the state wherein acoating film 91 having a thickness of 0.1 to 10 μm is formed selectivelyon a side surface of the piezoelectric/electrostrictive device 80 using,for example, SiO₂ added with a trace amount of TiO₂ through sputtering.

On the other hand, FIGS. 10(a) and (b) show the state wherein coatingfilms 92 having a thickness of 0.1 to 10 μm are formed on all thesurfaces of the piezoelectric/electrostrictive device 80 using, forexample, a siloxane solution according to the coating method. Thesiloxane solution is converted into a silica film through the sol-gelreaction. Even by the aforementioned other means using polysilazane, itis possible to form coating films composed of substantially silica only.

Now, the second to fourth production methods of thepiezoelectric/electrostrictive device according to the presentinvention, i.e. embodiments of the production methods including adiffusion joining process, will be described. Although a thin plate or athick plate in the form of a metal plate having a window portion is usedin the following embodiments, a thin plate or a thick plate having nowindow portions may also be used as described before.

First, the fourth production method of thepiezoelectric/electrostrictive device according to the present inventionwill be described with reference to FIGS. 21(a) to 21(g). FIGS. 21(a) to21(e) are diagrams for explaining one example of processes of the fourthproduction method of the piezoelectric/electrostrictive device accordingto the present invention, FIG. 21(f) is a perspective view showing oneexample of the piezoelectric/electrostrictive device to be produced, andFIG. 21(g) is a side view thereof. A piezoelectric/electrostrictivedevice 300 shown in FIGS. 21(f) and 21(g) comprises a pair of mutuallyconfronting thin plate portions 312 and a fixing portion 314 supportingthe pair of thin plate portions 312, wherein movable portions 320 areprovided at tip end portions of the pair of thin plate portions 312, themovable portions 312 have mutually confronting end surfaces 334, and apiezoelectric/electrostrictive element 378 is provided on each of thethin plate portions 312.

The production processes will be described. First, as shown in FIG.21(a), a thin plate 371 that becomes thin plate portions 312 later, andone thin plate 372 (two or more may be provided) that has a windowportion 341 and becomes movable portions 320 and parts of fixingportions 314 later, are preliminarily joined with the thin plate 371placed on an upper side to form a preliminary stacked body, then joinedtogether by diffusion joining to prepare an intermediate joined body 373a (see FIG. 21(b)). Similarly, a thin plate 371 and a thin plate 372 arepreliminarily joined with the thin plate 372 placed on an upper side toform a preliminary stacked body, then joined together by diffusionjoining to prepare an intermediate joined body 373 b (see FIG. 21(b)).The thin plates 371, the thin plates 372, and thin plates 374 referredto hereinbelow are metal plates of, for example, 18Cr-8Mo, and havethicknesses of, for example, 60 μm (thin plate 371), 70 μm (thin plate372), and 150 μm (thin plate 374). The diffusion joining method forjoining the thin plates by diffusion joining after the formation of thepreliminary stacked body will be described in detail later.

Then, as shown in FIG. 21(b), three thin plates 374 (there is nolimitation in number if it is no less than one) that each have a windowportion 343 and that become the fixing portions later, are sandwichedbetween the intermediate joined body 373 a and the intermediate joinedbody 373 b, and the intermediate joined body 373 a, the intermediatejoined body 373 b, and the thin plates 374 are preliminarily joined toform a preliminary stacked body, then joined together by diffusionjoining to prepare a joined body 376 (see FIG. 21(c)).

Subsequently, as shown in FIG. 21(c), separately preparedpiezoelectric/electrostrictive elements 378 are disposed by adhesion onboth outer surfaces of the joined body 376, i.e. on the thin plates 371located at the lowermost layer and the uppermost layer, at positionscorresponding to window portions 342 of the thin plates 372, thereby toprepare an original piezoelectric/electrostrictive device 377 (see FIG.21(d)). Then, as shown in FIG. 21(e), the originalpiezoelectric/electrostrictive device 377 is cut along cutting lines 369so that eight individual piezoelectric/electrostrictive devices 300described above can be obtained.

The third production method of a piezoelectric/electrostrictive deviceaccording to the present invention follows the aforementioned fourthproduction method of the piezoelectric/electrostrictive device accordingto the present invention. Specifically, the third production method ofthe piezoelectric/electrostrictive device according to the presentinvention is a production method of a piezoelectric/electrostrictivedevice that comprises a pair of mutually confronting thin plateportions, and a fixing portion supporting the pair of thin plateportions, wherein one or more piezoelectric/electrostrictive elementsare disposed on at least one of the pair of thin plate portions, and themovable portions 320 are removed from the piezoelectric/electrostrictivedevice 300 shown in FIGS. 21(f) and 21(g). The production processesfollow the aforementioned processes shown in FIGS. 21(a) to 21(e) exceptthat the thin plates 372 are not handled.

Referring now to FIGS. 13 to 16, another example of the fourthproduction method of a piezoelectric/electrostrictive device accordingto the present invention will be described. The aforementioned processesshown in FIGS. 21(a) to 21(e) are processes for obtaining eightpiezoelectric/electrostrictive devices as an example. On the other hand,the following processes are processes for obtaining 160piezoelectric/electrostrictive devices as an example. In the processesshown in FIGS. 21(a) to 21(e), a plurality of (eight)piezoelectric/electrostrictive devices are produced so as to be arrayedin one direction (lateral direction in the figure), while, in theprocesses shown in FIGS. 13 to 16, piezoelectric/electrostrictivedevices are produced so as to be arrayed in two directions (in thefigure, 20 in lateral direction and 8 rows in vertical direction;20×8=160).

First, two thin plates 71 and two thin plates 72 are prepared eachobtained by processing, for example, a SUS304 thin plate by means of thepunching method using dies, or the chemical etching method. As shown inFIG. 13(a), each thin plate 71 is a metal plate that has window portions41 in predetermined positions, has a predetermined shape with athickness of, for example, 40 μm, and becomes thin plate portions later.Each thin plate 72 is a metal plate that has a shape corresponding tothe shape of the thin plate 71, has a thickness of, for example, 50 μm,has window portions 41 and window portions 42 in predeterminedpositions, and becomes movable portions and parts of fixing portions.Then, one of the thin plates 71 and one of the thin plates 72, and theother thin plate 71 and the other thin plate 72 are preliminarily joinedat four corners thereof using an adhesive agent, thereby to prepare twopreliminary stacked bodies 73 a and 73 b. The preliminary stacked body73 a has a stacked structure wherein the thin plate 71 is placed on anupper side, while the preliminary stacked body 73 b has a stackedstructure wherein the thin plate 72 is placed on an upper side (FIG.13(b) shows the preliminary stacked body 73 b). The predeterminedpositions of the thin plates designating the positions of the formationof the window portions represent positions corresponding to eight rowsin the vertical direction like the windows 41 and 42 shown in FIGS.13(a) and (b). Later-described window portions follow this.

The obtained two preliminary stacked bodies 73 a and 73 b are formedinto intermediate joined bodies 79 a and 79 b by joining thepreliminarily joined thin plates 71 and 72 through diffusion joining. Asshown in FIG. 17, the diffusion joining is carried out by placing, forexample, the preliminary stacked body 73 a between pressure dies 181made of graphite, sandwiching pressure plates 182 made of MgO of 80% ormore purity between the preliminary stacked body 73 a and the pressuredies 181, and pressing the preliminary stacked body 73 a by the pressuredies 181. The pressing condition is such that, for example, a pressingtemperature is 850° C., a pressing time is 30 minutes, a pressingatmosphere is 2×10⁻⁴ Torr, and a pressing pressure is 1.25 MPa. Thediffusion joining method and the condition thereof described here arethe same as those in the aforementioned and below-described diffusionjoining processes.

Then, as shown in FIG. 14(a), a plurality of thin plates 74 are stacked,to a predetermined thickness, between the obtained two intermediatejoined body 79 a (upper side) and intermediate joined body 79 b (lowerside), and preliminarily joined at four corners thereof by an adhesiveagent, thereby to prepare a preliminary stacked body 75, as shown inFIG. 14(b). In FIG. 14(a), each of the intermediate joined bodies 79 aand 79 b exposes the surface on the side of the thin plate 72 of thejoined thin plates 71 and 72. Each thin plate 74 is a metal plate madeof SUS304, which is the same as the thin plates 71 and 72, obtainedthrough processing by means of the punching method using dies, or thechemical etching method, having a thickness of, for example, 200 μm,having a shape corresponding to the thin plates 71 and 72, and havingthe window portions 43 in predetermined positions. The thin plates 74become the fixing portions later.

The obtained preliminary stacked body 75 is formed into a joined body 76by joining the preliminary joined intermediate joined bodies 79 a and 79b and thin plates 74 through diffusion joining. Then, as shown in FIG.15(a), predetermined positions of the obtained joined body 76 (portionslocated on the thin plate 71 and corresponding to positions of windows(openings) that exist at the window portions 42 of the thin plate 72,but do not exist at the window portions 41 of the thin plate 71) are setas adhesive agent applying portions 44, and an adhesive agent is appliedthereto by the screen printing method, then separately preparedpiezoelectric/electrostrictive elements 78 are placed on the adhesiveagent applying portions 44, and the adhesive agent is cured to fix thepiezoelectric/electrostrictive elements 78, thereby to obtain anoriginal piezoelectric/electrostrictive device 77, as shown in FIG.15(b). Although not shown, the piezoelectric/electrostrictive elements78 are also attached to the other of the thin plates 71 exposed at bothsurfaces of the joined body 76.

As the formation method of the piezoelectric/electrostrictive elements,there can be adopted, apart from the aforementioned method using theadhesion, a method of forming piezoelectric/electrostrictive elementsdirectly on each thin plate 71 using the film formation technique suchas the sol-gel method, sputtering, CVD, laser ablation, or plasmawelding.

Then, as shown in FIG. 16, the obtained originalpiezoelectric/electrostrictive device 77 is cut perpendicularly to alongitudinal direction of the window portions 41, 42 and 43 (lateraldirection in the figure) along shown cutting lines 69, so thatindividual piezoelectric/electrostrictive devices can be obtained(although not clearly shown in the figure, there are 21 cutting lines 69extending vertically in the figure, so that, by cutting, the originalpiezoelectric/electrostrictive device 77 is divided into 20piezoelectric/electrostrictive devices per lateral row in the figure).

Now, a second production method for making thepiezoelectric/electrostrictive device according to the present inventionwill be described. The second production method is a production methodfor making a piezoelectric/electrostrictive device that comprises a thinplate portion, and a fixing portion supporting the thin plate portionand formed with a cavity inside, wherein one or morepiezoelectric/electrostrictive elements are disposed on the thin plateportion in a position corresponding to the cavity of the fixing portion.As one example of the piezoelectric/electrostrictive device, a dropletdischarging device is cited and will be described based on productionprocesses shown in FIGS. 18(a) to (c).

A droplet discharging device 170 comprises a thin plate portion 412, anda fixing portion 414 supporting the thin plate portion 412 and formedwith a pressure chamber 161 (cavity) inside, wherein onepiezoelectric/electrostrictive element 178 is disposed on the thin plateportion 412 in a position corresponding to the pressure chamber 161 ofthe fixing portion 414.

First, a thin plate 171 that becomes the thin plate portion 412 later, athick plate 172 (at least a thin plate of one layer may be stacked) thathas a window portion 141 of a predetermined shape and becomes the fixingportion 414 later, and a thick plate 173 formed with through holes 142of a predetermined shape are prepared, then integrated through diffusionjoining after preliminary adhesion, thereby to obtain a joined body 174.The window portion 141 serves as the pressure chamber 161 (cavity) forpressurizing droplets, and the through holes 142 serve as a liquidintroducing port 162 for introducing a liquid into the pressure chamber,and a liquid discharging port 163 for discharging the liquid from thepressure chamber. Then, the piezoelectric/electrostrictive element 178is fixed onto the thin plate 171 of the joined body 174 by an adhesiveagent in a position corresponding to the window portion 141, so that thedroplet discharging device 170 can be obtained.

In the aforementioned example, the description has been given about thedroplet discharging device having only one cavity. However, in thediffusion joining method according to the present invention, sincedeformation of a member to be joined can be suppressed, even whenproducing a droplet discharging device having many cavities disposed,dispersion in discharge amounts between the respective cavities causedby position shift can be suppressed, so that the droplet dischargingdevice can be suitably used.

EXAMPLE

Hereinbelow, the first and second piezoelectric/electrostrictive devicesaccording to the present invention, i.e. thepiezoelectric/electrostrictive devices having the coating films, will bedescribed based on examples. However, the present invention is notlimited to those examples.

First, a ceramic stacked body was obtained from ceramic powdercontaining zirconia as a main component by the ceramic green sheetstacking method. Then, on the surfaces of the ceramic stacked body,piezoelectric/electrostrictive elements were formed using lead zirconatetitanate (piezoelectric/electrostrictive layers) and platinum(electrodes) by the screen printing method. Then, by making up the shapethrough the wire saw processing, 104 piezoelectric/electrostrictivedevices each being the same as the piezoelectric/electrostrictive device10 shown in FIG. 3 were obtained. Among them, 42 devices were used assamples B.

Then, 42 devices (corresponding to the samples B) of the obtainedpiezoelectric/electrostrictive devices were dipped in a polysilazanesolution (N310 produced by Clariant International Ltd.) to form coatingfilms, then were subjected to a heat treatment at 490° C. for 30minutes, thereby to prepare 20 piezoelectric/electrostrictive deviceswith silica films formed on all the surfaces thereof. These were used assamples A. The thickness of the silica film was 1 μm.

Similarly, 20 devices (corresponding to the samples B) of the obtainedpiezoelectric/electrostrictive devices were dipped in a fluorocarboncoating flux solution (obtained by diluting FC722 produced by Sumitomo3M Co. Ltd. 50 times using a solvent PF5060 produced by Sumitomo 3M Co.Ltd.) to form coating films, then were heated to dry at 120° C. for 30minutes, thereby to prepare 20 piezoelectric/electrostrictive deviceswith fluorocarbon coating films formed on all the surfaces thereof.These were used as samples C. The thickness of the fluorocarbon coatingfilm was 1 nm.

Temperature Characteristic Test

The sample A (one device) was placed on a hot plate and heated, then, bychanging the temperature, displacement in response to an input at therespective temperatures was measured using a laser Doppler velocitymeter (VL10 produced by Sony Corporation) (Example 1). The input was30±30 V in the form of 1 kHz sin wave, and the temperature was changedto 25° C., 70° C., 100° C. and 110° C. The sample B was also tested inthe same manner (Comparative Example 1). The result is shown in FIG. 7.

Cleanliness Evaluation

Pure water and the sample A (one device) were put into a fully washedcontainer, and ultrasonic cleaning (frequency: 68 kHz) was performed forthree minutes. Thereafter, the number of particles existing in the purewater in the container was measured using a particle counter (KL-26produced by Rion Co., Ltd.). The result was several particles of 0.5 μmor greater per milliliter. The sample B was also tested in the samemanner. The result was several hundred particles of 0.5 μm or greaterper milliliter, and thus it was about 100 times the sample A.

Durability Test 1

A sealed container (length 260 mm×width 190 mm×height 90 mm) containingan ammonium sulfate saturated salt solution was put into a lowtemperature incubator (SLV-11 produced by Isuzu Co., Ltd.) set to 40°C., thereby to provide a constant-temperature constant-humidityenvironment (40° C., 85±5% R. H. (Relative Humidity)). Then, the samplesA (20 devices) were put into the sealed container and operatedcontinuously, thereby to examine durability thereof. The input was 30±30V in the form of 1 kHz sin wave. The samples B were also tested in thesame manner. The result was that, in case of the samples B, five devicescaused short circuit due to migration after a lapse of 100 hours, while,in case of the samples A, there was no occurrence of short circuit evenafter a lapse of 1000 hours.

Durability Tests 2, 3 and 4

By changing the solution, the temperature, and the humidity, the samplesA (20 devices) and the samples B (20 devices) were tested in the samemanner as in Durability Test 1 in constant-temperature constant-humidityenvironments like (Test 2) in case of a potassium bromide saturated saltsolution at 20° C. and 84% R.H., (Test 3) in case of a sodium carbonatesaturated salt solution at 25° C. and 87% R.H., and (Test 4) in case ofa sodium bromide saturated salt solution at 40° C. and 55% R.H. In allthe environments, the failure occurrence rates were lower in case of thesamples A as compared with the samples B.

Durability Test 5

The samples A (20 devices) were operated continuously in environment at85° C. and 85% R.H. (Relative Humidity) using a constant-temperatureconstant-humidity bath (PH-1K produced by Espec Corporation), thereby toexamine durability thereof at high temperature and high humidity. Theinput was 30±30 V in the form of 1 kHz sin wave. The samples B were alsotested in the same manner. The result was that, in case of the samplesB, 12 devices caused short circuit due to migration after a lapse of 100hours, while, in case of the samples A, only three devices caused shortcircuit even after a lapse of 500 hours.

Durability Test 6

The samples A (20 devices) were operated continuously in a dry nitrogenatmosphere using an inert oven (IPH-201 produced by Espec Corporation),thereby to examine the rate of capacitance change per lapse of a time soas to confirm durability thereof over a long term (Example 2). The inputwas 30±30 V in the form of 1 kHz sin wave. The change rate wascalculated using the average of capacitances of 20 samples. The samplesC were also tested in the same manner (Comparative Example 2). Theresult is shown in Table 1. TABLE 1 Driving Time 100 hours 1000 hours10000 hours Example 2   0% −0.5% −3% Comparative −1%   −5% −30% Example2

As clear from the aforementioned description, in accordance with thepiezoelectric/electrostrictive device according to the present inventionand its production method, further weight reduction can be achieved,greater displacement can be ensured, speed-up of the displacingoperation (higher resonance frequency) can be achieved, it is notsusceptible to influence of harmful vibration, faster response is madepossible, mechanical strength is enhanced, it is excellent inhandleability, displacement excellent in controllability can be achievedthat follows an applied electric field irrespective of change intemperature of environment of use or an element itself, or even upon useat high temperatures, so that high reliability can be ensured over along term.

Further, the piezoelectric/electrostrictive device described above canbe used as an active element such as a transducer, an actuator, afrequency region functioning component (filter), a transformer, avibrator or a resonator for communication or power, an oscillator or adiscriminator, or as a sensor element for various sensors such as anultrasonic sensor, an acceleration sensor, angular velocity sensor, animpact sensor and a mass sensor. Particularly, it can be suitably usedfor various actuators used in mechanisms for adjusting displacement,position and angle of various precision components of optical equipment,precision equipment etc.

The piezoelectric/electrostrictive element according to the presentinvention is excellent in temperature characteristic, and low inparticle occurrence rate, and has high durability, so that it ispreferably used as a component of the aforementionedpiezoelectric/electrostrictive device, and further, it can be used foractuator portions of electrical, electronic products etc. exposed instrict environment of use. The electrical, electronic products etc.using the piezoelectric/electrostrictive element according to thepresent invention can achieve longer duration of life to improvecompetitive strength thereof.

1. A method of producing a piezoelectric/electrostrictive devicecomprising a pair of mutually confronting thin plate portions, and afixing portion supporting said pair of thin plate portions, whereinmovable portions are provided at tip end portions of said pair of thinplate portions, said movable portions have mutually confronting endsurfaces, and one or more piezoelectric/electrostrictive elements aredisposed on at least one of said pair of thin plate portions, whereinsaid method comprises steps of forming said one or morepiezoelectric/electrostrictive elements on said at least one of saidpair of thin plate portions, then covering at least both side surfacesof said thin plate portions and said one or morepiezoelectric/electrostrictive elements with coating films made of a lowthermal expansion coefficient material by a film formation method.
 2. Amethod of producing a piezoelectric/electrostrictive device according toclaim 1, wherein said film formation method is a method selected fromthe group consisting of sticking of a filmy plate, coating, dipping,sputtering, CVD, and laser ablation.
 3. A method of producing apiezoelectric/electrostrictive element according to claim 1, whereinsaid method further comprises covering at least a pair of side surfacesparallel to a displacing direction with coating films made ofsubstantially SiO₂ only and each having a thickness of 0.1 μm or greaterwith a film formation method.
 4. A method of producing apiezoelectric/electrostrictive element according to claim 3, whereinsaid film formation method is a coating method or a dipping method usinga polysilazane.
 5. A method of producing apiezoelectric/electrostrictive device comprising a thin plate portionand a fixing portion supporting said thin plate portion and formed witha cavity inside, wherein one or more piezoelectric/electrostrictiveelements are disposed on said thin plate portion in a positioncorresponding to the cavity of said fixing portion, wherein said methodcomprises steps of: preparing a joined body by joining a thin plate thatbecomes said thin plate portion later, and a thick plate that comprisesat least one layer and becomes said fixing portion later, throughdiffusion joining; and forming said one or morepiezoelectric/electrostrictive elements on said thin plate of saidjoined body.
 6. A method of producing a piezoelectric/electrostrictivedevice comprising a pair of mutually confronting thin plate portions anda fixing portion supporting said pair of thin plate portions, whereinone or more piezoelectric/electrostrictive elements are disposed on atleast one of said pair of thin plate portions, wherein said methodcomprises steps of: preparing a joined body by joining thin plates thatbecome said thin plate portions later, and one or more thin plates orthick plates that become said fixing portion later, through diffusionjoining; disposing said one or more piezoelectric/electrostrictiveelements on at least one of said thin plates of said joined body toprepare an original piezoelectric/electrostrictive device; and cuttingsaid original piezoelectric/electrostrictive device to obtain individualpiezoelectric/electrostrictive devices.
 7. A method of producing apiezoelectric/electrostrictive device comprising a pair of mutuallyconfronting thin plate portions, and a fixing portion supporting saidpair of thin plate portions, wherein movable portions are provided attip end portions of said pair of thin plate portions, said movableportions have mutually confronting end surfaces, and one or morepiezoelectric/electrostrictive elements are disposed on at least one ofsaid pair of thin plate portions, wherein said method comprises stepsof: preparing intermediate joined bodies each by joining a thin platethat becomes said thin plate portion later, and one or more thin platesor thick plates that become said movable portion and part of said fixingportion later, through diffusion joining; preparing a joined body byjoining said intermediate joined bodies and one or more thin plates orthick plates that become said fixing portion later, through diffusionjoining; disposing said one or more piezoelectric/electrostrictiveelements on at least one of said thin plates of said joined body toprepare an original piezoelectric/electrostrictive device; and cuttingsaid original piezoelectric/electrostrictive device to obtain individualpiezoelectric/electrostrictive devices.
 8. A method of producing apiezoelectric/electrostrictive device according to claim 5, wherein a0.2% proof stress at 800° C. of said thin plate or thick plate is 75 MPaor greater.
 9. A method of producing a piezoelectric/electrostrictivedevice according to claim 5, wherein said diffusion joining is a joiningmethod in which a member to be joined is placed between two pressuredies and pressed at a predetermined temperature, and in which saidmember to be joined is pressed in the state where pressure plates madeof the same material as that of said member to be joined and appliedwith solid lubricant are interposed between said pressure dies and saidmember to be joined.
 10. A method of producing apiezoelectric/electrostrictive device according to claim 9, wherein saidsolid lubricant contains at least hexagonal boron nitride.
 11. A methodof producing a piezoelectric/electrostrictive device according to claim5, wherein said diffusion joining is a joining method in which a memberto be joined is placed between two pressure dies and pressed at apredetermined temperature, and in which said member to be joined ispressed in the state where ceramic pressure plates having a thermalexpansion coefficient that is within a range of ±30% relative to athermal expansion coefficient of said member to be joined, areinterposed between said pressure dies and said member to be joined. 12.A method of producing a piezoelectric/electrostrictive device accordingto claim 11, wherein said ceramic pressure plates are made of calciumoxide or magnesium oxide of 80% or more purity.
 13. A method ofproducing a piezoelectric/electrostrictive device according to claim 5,wherein said thin plate or thick plate that becomes at least part ofsaid fixing portion later, is formed with a window portion in advance.